This invention relates generally to electro-optical devices such as liquid crystal devices.
Liquid crystal displays use a spatial light modulator (SLM) made up of a top plate and a substrate which surround a liquid crystal material. Conventionally, the region for the liquid crystal material is defined by spacer balls which may be distributed over the substrate. In addition, it is To known to fabricate insulating spacers directly on a silicon substrate. The function of the spacers is to maintain the distance between the top plate and the substrate and to define the region for the liquid crystal.
Liquid crystal devices using liquid crystal over a silicon substrate (LCOS) technology may form large screen projection displays or smaller displays (using direct viewing rather then projection technology). Typically, the liquid crystal material is suspended over a thin passivation layer. A glass plate with an indium tin oxide (ITO) layer covers the liquid crystal, creating the liquid crystal unit sometimes called a cell. The glass layer is typically suspended over the liquid crystal by a gasket that surrounds the cell array.
A silicon substrate may define a large number of pixels. Each pixel may include semiconductor transistor circuitry in one embodiment. The pixel may have a top reflective layer. An electrical potential may be applied to an electro-optical material using the reflective layer. A transparent top plate may have an inner transparent conductive layer that acts as an electrode that works with the reflective layer. An electrical field generated at each pixel may alter optical characteristics of an electro-optical material between the silicon substrate and the top plate. For example, the polarization of light passing through the electro-optical material may be altered. As another example, the electro-optical material may change its light transmission characteristics.
In some cases a standoff may be used to support the top plate. However, it is desirable to maintain a constant cell gap, thereby creating a constant liquid crystal thickness over the entire area of a cell array. To varying degrees, the use of discrete spacers is a simple way of providing this spacing, but in many cases they may not provide a sufficiently constant cell thickness.
Another technique uses an oxide spacer which may be formed on top of the pixel mirrors. However, this technique has the disadvantage that it requires extra processing steps.
Thus, there is a continuing need for a way to improve the uniformity of the spacing between the top plate and substrate in electro-optical devices without unduly increasing processing overhead.